Fix unknown tags in comment tags, second try

Had to revert #256, because people are using this in Shopify:
```{% comment %} foo {% comment %} bar {% endcomment %} bla {% endcomment %}```, which failed with #256.

So back to my original ```unknown_tag``` approach. This solves my original issue, but unfortunately does not allow comments around syntax errors. If people really need that, they can use ```{% comment %} {% raw %} ... {% endraw %} {% endcomment %}``` I guess. Not a nice solution, but everything else would be much more complicated I think.

Please take another look @Sirupsen @dylanahsmith / cc @silverstreaked